linux-kernel - Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

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Message-ID: <20241211222257.GKZ1oQwZcSXSMXPvoY@fat_crate.local>
Date: Wed, 11 Dec 2024 23:22:57 +0100
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: "Nikunj A. Dadhania" <nikunj@....com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, kvm@...r.kernel.org, mingo@...hat.com,
	tglx@...utronix.de, dave.hansen@...ux.intel.com, pgonda@...gle.com,
	seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure
 TSC enabled guests

On Wed, Dec 11, 2024 at 04:01:31PM -0600, Tom Lendacky wrote:
> It could be any reason... maybe the hypervisor wants to know when this
> MSR used in order to tell the guest owner to update their code. Writing
> to or reading from that MSR is not that common, so I would think we want
> to keep the same behavior that has been in effect.

Ah, I thought you're gonna say something along the lines of, yeah, we must use
the HV GHCB protocol because of <raisins> and there's no other way this could
work.

> But if we do want to make this change, maybe do it separate from the
> Secure TSC series since it alters the behavior of SEV-ES guests and
> SEV-SNP guests without Secure TSC.

Already suggested so - this should be a separate patch.

It would be interesting to see if it brings any improvement by avoiding the HV
call... especially since RDTSC is used a *lot* and prominently at that in
sched_clock, for example.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette